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Nov. 22, 2011 -- Long-time coffee fans who drink four or more cups a day of caffeinated coffee may be reducing their endometrial cancer risk by 30%, a new study shows.

And if you prefer decaf, drinking two or more cups showed a trend toward reducing risk by about 22%. "Consistent with other reports, this study suggests that women who drink coffee, regular or decaf, are at reduced risk of endometrial cancer," says study researcher Edward Giovannucci, MD, ScD, professor of nutrition and epidemiology at Harvard School of Public Health.

Even so, Giovannucci says, "It's premature to recommend that women take up coffee to reduce the risk of endometrial cancer." He found a link, not cause and effect.

About 46,470 new cases of endometrial cancer and 8,120 deaths are expected in 2011, according to the American Cancer Society. The cancer occurs in the womb, or inner lining of the uterus.

Tracking Coffee Habits and Cancer Risk

Giovannucci evaluated more than 67,000 women enrolled in the long-running Nurses' Health Study to examine the link between coffee and endometrial cancer risk.

During the 26-year follow-up, there were 672 cases of endometrial cancer.

The researchers found a stronger protective effect among obese women, which has also been found in other research. Obesity is a risk factor for endometrial cancer.

The link between four or more cups of coffee and reduced risk of endometrial cancer was also stronger for:

past or current smokers

women past menopause

women not on menopausal hormone therapy

"Coffee seems to improve insulin resistance, which lowers insulin levels," Giovannucci says, and coffee and caffeine have also been shown to decrease estrogen levels.

"Both high estrogen and high insulin have been associated with higher risk of endometrial cancer," he says.

As for the stronger effect in smokers, he says, "We don't know why at this point."

It's not clear if the results would apply to those who drink coffee with sweeteners or cream, the researchers say. No information was given on how the women drank their coffee. Adding sugar or cream could offset any potential benefits.

It might be more than the caffeine providing the protection, Giovannucci says. "Coffee contains numerous compounds (outside of caffeine) that may improve insulin action in the body."

Coffee and Endometrial Cancer Risk: Second Opinion

The results are "intriguing," says Marji McCullough, ScD, RD, strategic director of nutritional epidemiology for the American Cancer Society. She reviewed the study but was not involved in it.

The finding about a weaker link in people who have never smoked needs more study, she says.

But McCullough agrees the study does suggest that a woman who currently drinks coffee may be receiving benefit in terms of reducing her risk of endometrial cancer.

"If not, there are other important ways to reduce the risk of endometrial cancer, such as maintaining a healthy body weight and staying physically active," she tells WebMD.